This invention relates to a silent chain used for power transmission, in applications such as the timing drive of automobiles or motorcycles, or as a chain drive in a general purpose engine, a diesel engine or an industrial machine or the like. More specifically, the invention relates to a silent chain used as a timing chain in an engine.
An example of a conventional silent chain a bush-type silent chain 1 is shown in FIGS. 3(A) and 3(B). In this silent chain 1, guide plates 2, link assemblies 3 and intermediate link plates 6 are disposed adjacent one another in the direction of the width of the chain, but shifted longitudinally so that they are interleaved. The interleaved members are articulably connected with one another by connecting pins 7.
The guide plates 2 do not have teeth, but each guide plate has a pair of pin holes 2a as shown in FIG. 4. Each link assembly 3 comprises two inner link plates 4 each having a pair of teeth 4a, which mesh with the teeth of a sprocket, a pair of bushing holes 4b, and a pair of bushings 5, each press-fit into a bushing hole 4b, as shown in FIGS. 5(A) and 5(B). In the link assembly 3, the two inner link plates 4 are integrally connected and fixed to each other by two bushings 5. Further, each intermediate link plate 6 includes a pair of teeth 6a, which mesh with the teeth of a sprocket, and a pair of pin holes 6b, as shown in FIG. 6.
In the inner link plate 4, as shown in FIG. 5(A) and FIG. 7, a pair of teeth 4a are formed, each having an outside surface portion 4c and an inside surface portion 4d, a pair of bushing holes 4b, into each of which a bushing 5 is press-fit and fixed, a back surface portion 4e formed on the side opposite to the side where the pair of teeth 4a is formed, and a pair of shoulder portions 4f, each of which connects an outside surface portion 4c to the back surface portion 4e. The broken line P in FIG. 7 is a pitch line, which passes through the centers of the bushing holes 4b. 
In the particular chain shown in FIG. 3(A), two intermediate link plates 6 are disposed centrally with respect to the width direction of the chain, link assemblies 3 are disposed on both outer sides of the intermediate link plates 6, and the guide plates 2 are disposed on both of the outermost sides of the chain. These elements are longitudinally shifted with respect to one another, and thereby interleaved, and are articulably connected to one another by connecting pins 7. In this case, the connecting pin 7 is press-fit into and fixed to the pin holes 2a of the guide plates 2 on both outermost sides in the chain, and in the intermediate link plates 6, the connecting pin 7 can extend through the pin holes 6b with play, or can be press-fit and fixed into the pin hole 6b. 
Since the link assembly 3 is composed of plural link plates 4, the problem arises that, if the inner link plates are not light in weight, the weight of the silent chain becomes excessive. Further, the rigidity of the teeth of the inner link plates is high. Thus, when the chain meshes with the teeth of a sprocket during operation, tooth surfaces of each inner link plate collide with tooth surfaces of the sprocket and the collision shock is dispersed to the entire inner link plate, generating meshing noises of large amplitude.
In the inner link plates 4, which form the link assembly 3, the bushing 5 is press-fit into, and fixed to, the bushing hole 4b. Thus, the bushing hole 4b, formed in the inner link plate 4, is a size larger than the pin hole 6b in the intermediate link plate 6. As a result, the distance between the bushing hole 4b and the adjacent outer side portion of the inner link plate 4 becomes smaller than the corresponding distance in the intermediate link plate 6. In this case, as shown in FIG. 7, the shortest distance kxe2x80x2 between the bushing hole 4b and the outside surface portion 4c, and the shortest distance lxe2x80x2 between the bushing hole 4b and the inside surface portion 4d are comparatively small. However, the shortest distance wxe2x80x2 between the bushing hole 4b and the tip of the tooth of the inner link plate is still relatively large. Accordingly, the distances lxe2x80x2, kxe2x80x2 and wxe2x80x2 become unbalanced.
When the bushing 5 is press-fit into the bushing hole 4b in the inner link plate 4 to form a link assembly, the inner link plate 4 is usually deformed in the areas where the distance between the bushing hole 4b and the outer side portion is small. Accordingly, strain is generated in the inner diameter of the press-fit bushing 5. Since the back surface portion 4e and the shoulder portions 4f of the inner link plate 4 have no direct relationship to the meshing of the link plate teeth with the teeth of a sprocket, it is possible to suppress the strain generated in the inner diameter of the press-fit bushing by increasing the shortest distance mxe2x80x2 between the bushing hole 4b, and the back surface portion 4e and the shortest distance nxe2x80x2 between the bushing hole 4b, and the shoulder portions 4f. However, in the teeth 4a, the distances kxe2x80x2 and lxe2x80x2 cannot be correspondingly increased without affecting the meshing relationship between the teeth 4a and the teeth of the sprocket. Thus, when the distances k and l are small, and the distance wxe2x80x2 is large, the inner diameter of the press-fit bushing 5 has different degrees of deformation in the areas corresponding to the distances kxe2x80x2 and 1xe2x80x2 and the area corresponding to the distance wxe2x80x2. As a result nonuniform strains are generated, and the inside of the bushing deviates from true roundness. This problem arises because the relationships between the shortest distances and the plate thickness of the inner link plate have not been noted. Thus, the distances kxe2x80x2 and lxe2x80x2 may be too small compared to the plate thickness of the inner link plate. However, these problems can arise even when the plate thickness is increased.
When the roundness of the inner diameter of the bushing press-fit into the bushing hole deteriorates, if the link assembly is incorporated into a chain, the interfacial pressure between the surface of the connecting pin and the inner periphery of the bushing becomes nonuniform, or the connecting pin makes contact with the inner periphery of the bushing only on one side. Consequently, wear of the connecting pin and bushing, and resulting wear elongation of the chain are both accelerated.
Accordingly, a general object of this invention is to overcome the problems associated with the above-described conventional silent chain.
A more specific object of the invention is to provide a silent chain in which wear of the connecting pins and bushings, and wear elongation of the chain can be suppressed.
According to one aspect of the invention, the silent chain comprises a link assembly including at least two inner link plates, each link plate having a pair of teeth and a pair of bushing holes, the link plates being connected and fixed to one another by a pair of bushings, each said bushing being press-fit into bushing holes of all of said link plates of the link assembly, wherein each of the teeth of the inner link plates has a through-hole approximately in its center.
According to the invention, the deformation of the inside of a press-fit bushing can be made more uniform compared to the deformation in the case of conventional inner link plate lacking the through-holes. Accordingly, deviation from roundness in the inside of the bushing can be prevented.
Further, the through-holes reduce the weight of the inner link plates, and as a result, a weight reduction of the a reduction in the overall weight of the silent chain can be realized. Because of this weight reduction, when the silent chain meshes with the sprocket teeth the shock energy due to the collision between the tooth faces of the inner link plates and sprocket is reduced, so that the occurrence of high-amplitude meshing noises can be prevented, and wear of the tooth faces can be suppressed.
In the silent chain according to the invention, each tooth of the pair of teeth of an inner link plate is adjacent a bushing hole and includes an outside surface portion and an inside surface portion. The inner link plate is preferably formed so that the shortest distance between the bushing hole and the through-hole, the shortest distance between the bushing hole and the outside surface portion, and the shortest distance between the bushing hole and the inside surface portion, are substantially equal, and the respective shortest distances are equal to, or greater than, the plate thickness of the inner link plate. When the above-mentioned relationships between the shortest distances and the plate thickness are satisfied, deformation of the inner link plate during press-punching can be prevented. Moreover, reduction in vertical accuracy of the bushing hole with respect to the inner link plate, thus the reduction in vertical accuracy of the press-fit bushing therewith can be prevented. Further, in the press-fit bushings, deformation of the inner peripheries of the bushings becomes nearly uniform, so that deviations from roundness of the inner peripheries of the bushings can be prevented. As a result, since biased contact between the connecting pins and the inner peripheries of the bushings can be prevented, and at the same time interfacial pressure between the connecting pins and the inner peripheries of the bushings becomes uniform, wear of the connecting pins and bushings, and wear elongation of the chain, can be suppressed.